This invention relates to an infrared optical fiber capable of retaining its infrared transmission property stably for a long time.
Infrared optical fibers which use core materials of halogenides such as TlBr, TlCl, AgBr, AgCl, CsI and CsBr are capable of transmitting infrared lights of longer wavelengths than optical fibers using materials such as of quartz glass and, therefore, are advantageously used as transmission means such as for CO.sub.2 lasers and CO lasers in long wavelength regions. They nevertheless have a disadvantage that their infrared transmission property tend to degrade rapidly.
The halogenides mentioned above have such narrow zones of elasticity that the optical fibers made thereof, upon exposure to a very slight bending strength, will readily undergo plastic deformation. The plastic deformation coarsens the core-clad interface of the optical fibers and thereby the rugged interface induces scattering loss of infrared rays and degradation of transmissivity.
The aforementioned infrared optical fibers are manufactured by the extrusion method or the pulling method and, consequently, the produced cores assume a polycrystalline structure. With lapse of time, therefore, the cores allow growth of microcrystals and undergo gradual internal change in structure. At the same time, the core-clad interface is gradually disturbed so much as to induce a decline in the transmissivity of the optical fibers.